The present invention relates to the recovery of oil from subterranean reservoirs and more particularly relates to a process of improving the sweep efficiency of a waterflood, oil recovery operation.
The use of a "waterflood" operation to recover oil from subterranean reservoirs is well known in the petroleum production art. A typical waterflood involves injecting an aqueous flooding liquid, e.g., water, into a reservoir to drive oil through the reservoir toward one or more production wells from which it can be produced to the surface. Unfortunately, the injected water has a tendency to channel through certain portions of the reservoir while completely bypassing others. This inability of the water to "sweep" substantial areas within the reservoir seriously affects the ultimate recovery of oil and detracts from the economical attractiveness of such operations. Accordingly, the need to improve the sweep efficiency of waterfloods has long been apparent.
Early research revealed that poor sweep efficiency can be attributed to several factors. One is the natural tendency of a liquid to flow in the path of least resistance; hence, the water flows through the highly permeable portions of the reservoir more readily than through the less permeable portions. Another factor involves the differences between the mobilities of the injected water and the oil in the reservoir. In both instances, the sweep efficiency of a waterflood can be improved by increasing the viscosity of the injected water.
Past techniques for increasing the viscosity of flood water have included incorporating water-soluble thickening additives into the water. Additives that have been suggested for this purpose include a wide variety of naturally occurring gums, sugars, and polymers. One of the most promising groups of such thickeners is the ionic polysaccharides, particularly the polysaccharide prepared by employing bacteria of the genus Xanthomonas and commonly referred to as polysaccharide B-1459.
This invention provides a method whereby the viscosity of a polysaccharide solution may be increased with no additional use of chemical.